Heparin causes thrombocytopenia in 1 - 3 per cent of patients and leads to thrombosis in about half of these. There have been several laboratory tests to diagnose this problem, but none of them is perfect and some are very labor intensive. Recently a new assay method has been published that offers the promise of high sensitivity and specificity but with relatively little labor. In order to collect a sufficient number of patient samples to study this new assay, we have collaborated with physicians at Washington Hospital Center, where heparin-induced thrombocytopenia (HIT) is much more common than here. Thirty-four patients have been accrued from the Hospital Center and 21 from NIH with a spectrum of clinical probability of actually having HIT. All of the laboratory testing has been completed. The newly introduced assay was neither sensitive or specific enough to be implemented and so has been abandoned. The labor-intensive assay proved to be no more sensitive or specific than the laborious "gold standard" method. We are awaiting final confirmation of the clinical scoring before preparing the data for publication.